#include <bits/stdc++.h>
using namespace std;

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define BIT(x,n) bitset<n>(x)
#define PI 3.14159265358979323846

typedef long long ll;
typedef pair<int,ll> P;
typedef pair<ll,P> PP;

//-----------------------------------------------------------------------------

int n,m;
ll a[100000];

bool ok(ll mid) {
	int l=0,r=n-1,cnt=0;
	while(l<r) {
		while(l<r-1&&a[l]+a[r]<mid) r--;
		if(a[l]+a[r]>=mid) cnt++;
		l++,r--;
	}
	//cout<<"mid="<<mid<<",cnt="<<cnt<<endl;
	return cnt>=m;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin>>n>>m;
	REP(i,n) cin>>a[i];
	ll k=a[0];

	sort(a,a+n,greater<int>());

	ll ub=INF,lb=0;
	while(ub-lb>1) {
		ll mid=(ub+lb)/2;
		if(ok(mid)) lb=mid;
		else ub=mid;
	}
	//cout<<ub<<' '<<lb<<endl;
	sort(a,a+n);
	int ans,idx=lower_bound(a,a+n,lb-k)-a;
	if(idx==n) ans=-1;
	else ans=lb-a[idx];
	cout<<ans<<endl;
	//cout<<idx<<' '<<a[idx]<<endl;

	return 0;
}